High Frequency and Wideband Modulated Signal Generation Using Frequency Doublers
This paper proposes a digital predistortion (DPD) scheme devised to improve the quality of wideband digitally modulated signals generated using frequency doubler based high frequency sources. The proposed DPD consists of two modules, which are configured to tackle the nonlinearity exhibited by frequency doublers when driven with high peak-to-average-power ratio (PAPR) signals. The first module is a memoryless (ML) polynomial describing function with a square root basis to mitigate the squaring transformation of the input envelope signal to the frequency doubler. The second module consists of a pruned Volterra-series based model, which is incorporated to mitigate residual distortions. The proposed DPD scheme was validated by linearizing an active frequency doubler with its output centered at 25 GHz when stimulated with orthogonal frequency division multiplexing signals with modulation bandwidths of 160 MHz and 320 MHz, and PAPRs of 8.6 dB. When both modules of the proposed DPD scheme are set to be ML with nonlinearity orders of 4 and 6, the linearized frequency doubler exhibited normalized mean squared errors (NMSEs) of -28 dB, -26 dB and adjacent channel power ratios (ACPRs) of 50/47 dBc, 42/39 dBc for modulation bandwidths of 160 MHz and 320 MHz, respectively. By only adding memory distortion terms to the second module, the NMSEs significantly decreased from -28 dB to -35 dB for the 160 MHz case and from -26 dB to -32 dB for the 320 MHz case. For both cases, the ACPRs became more balanced after adding memory terms to the second module.